Usually, since UV radiation has higher energy than that of another light of sunlight, it has negative health effects on the human body and others. For this reason, UV radiation is desired to be shielded. When even UV-resistant glass is used for a long duration, discoloration is produced, and the content inside the glass is further deteriorated in discoloration caused by UV radiation. Specifically, polymeric resins exhibit large degradation caused by UV radiation.
Usually, a polymer film produces cleavage of molecular chains via photooxidation reaction upon exposure to UV radiation under the existence of oxygen, whereby degradation in strength, rise in haze, and lowering of transparency and color tone caused by yellowish change or the like (deterioration in UV radiation). UV radiation of sunlight has a wavelength of 295-400 nm, and light energy in this wavelength region is almost equal to the bonding energy of C, H or O. Therefore, when a plastic molding mainly having the bonding of C, H or O is exposed to UV radiation, such a bonding is broken, whereby degradation of resin, discoloration and lowering in mechanical strength tend to be produced. Accordingly, the plastic molding cannot be used stably in an outdoor location for a long duration. For this reason, generally well known is a technique by which a light stabilizer is blended in a polymer film to improve weather resistance of the resulting polymer film.
The light stabilizer is a stabilizer used for the purpose of inhibiting photooxidation reaction of the polymer film caused by UV radiation, and as the light stabilizer, a UV absorbent, quencher, and HALS (Hindered Amine Light Stabilizer) are well known.
The UV absorbent is a light stabilizer which absorbs UV radiation, and discharges energy absorbed in the molecule by making the energy to be lower energy in the form of heat, phosphorescence, or fluorescence, and as the UV absorbent, a benzophenone type, a benzotriazol type, a benzoate type, a cyanoacrylate type and so forth are practically available.
The quencher is a light stabilizer by which one in which a chromophoric group (mainly an unsaturated hydrocarbon and its compound) being on a ground state absorbs UV radiation so as to become an excited state is returned into the original ground state, and as the quencher, Ni compounds are employed.
The HALS is a light stabilizer which traps an alkyl radical, a peroxy radical and so forth produced via exposure to UV radiation to inhibit photooxidation reaction, and the HALS is a compound having a hindered piperidine moiety.
However, even though a light stabilizer such as a UV absorbent or the like is contained in a polymer film, influence of UV radiation can not be sufficiently removed from the surface, whereby deterioration on the surface of the polymeric resin can not be suppressed. Further, in order to obtain sufficient weather resistance, a sufficient amount of a light stabilizer is desired to be contained, but the polymeric resin is exposed to an environment of heat and water, bleed-out, sublimation and so forth are generated, whereby the light stabilizer is reduced, resulting in lowering in weather resistance, lowering in transparency and rise in haze, leading to insufficient aging stability. Further, since a light stabilizer is expensive, a large cost-up results.
As another method of enhancing weather resistance, there is a method of coating a layer containing a UV absorbing material composed of an organic substance or inorganic particles (refer to Patent Documents 1 and 2). This is an effective means, but in the case of outdoor use, the surface of the coating film is exposed to rain water, oxygen in the air, or pollution substances, and exposed to UV radiation since there is no sufficient UV absorbent on the surface. Therefore, deterioration caused by UV radiation is generated, and there appears a large problem such as obstacles such as coloring such as yellowish color, lowering in transmittance and rise in haze.
Further, in the case of at least one of a UV absorbent and a hindered amine based light stabilizer, disappearance of bleed-out produced by the UV absorbent or the like is seen, resulting in insufficient weather resistance. Further, a large amount of UV absorbent is to be provided in order to obtain sufficient UV absorbability. However, when increasing a UV absorbent amount, absorbing occurs in not only a UV wavelength range, but also even a short wavelength range of visible light, resulting in lowering in transmittance from the initial to visible light region.
In cases where inorganic particles are used as a light stabilizer (Patent Document 1), they are more stable than an organic light stabilizer with respect to UV radiation, heat and so forth. However, similarly to an organic UV absorbent, influence of UV radiation to the surface can not be sufficiently eliminated, and deterioration caused by UV radiation, on the surface of a resin layer containing inorganic particles, can not be inhibited. Therefore, lowering in rise in haze, yellowish change, and visible light transmittance results. Further, inorganic UV absorbents should have an average particle diameter of 100 nm or less in order to maintain transparency and a clear property in the visible light region. When the particle diameter is small, the particles exhibit high surface energy, and coagulation of particle-to-particle is easy to be generated, whereby an aggregate thereof produces scattering of light in the visible light region, and the resulting coated film tends to lose a clear property. Further, in order to obtain sufficient UV shielding performance for protection of a resin support, coagulation of particles is further produced, since a necessary amount of inorganic UV shielding agent is larger than that of an organic UV absorbent. Further, inorganic particles tend to absorb UV radiation to activate surrounding water, oxygen and so forth (photocatalytic function). Therefore, in the case of outdoor use, water such as rain water or the like is activated with UV radiation and inorganic particles, whereby the active species decompose organic components of the coating film and the resin support, resulting in generation of a peeled coating film, rise in haze, yellowish change and so forth.
As another method of improving weather resistance, there is a method of providing a UV radiation reflective layer by a dielectric laminate in which a low refractive index layer and a high refractive index layer are alternately laminated (for example, Patent Documents 3 and 4). In the case of this method, since the dielectric is made of an inorganic substance, a UV shielding film composed of a dielectric laminate is difficult to be deteriorated even though it is exposed to UV radiation, an atmospheric pollutant, rain water or the like, leading to largely effective shielding of UV radiation for a long duration. However, in order to shield 90% or more of UV radiation, a certain amount of film thickness should be provided, resulting in high cost. Further, since at least several percent of UV radiation is transmitted, the resin support is degraded because of this several percent of UV radiation in the case of outdoor use for a long duration (at least a few years).